Field of the Invention
This invention relates to the field of mold release agents and methods of using them. More particularly, this invention relates to mold release agents for flexible polyurethane foam and to methods of using them particularly for the release of flexible polyurethane integral-skin foam.
There are three broadly acknowledged classes of polyurethane foam: rigid, semi-rigid, and flexible. This invention concerns the latter class. More specifically, it concerns a type of flexible polyurethane foam known as integral-skin foam.
Methods of preparing flexible polyurethane foam and integral-skin varieties thereof as well as specific formulations are well-known in the art. See, for example, "Polyurethane Foams" edited by T. T. Healy, AIRI, London, ILIFFE Books, Ltd., (1963), and "Plastics Engineering Handbook", Third Edition, Reinhold Publishing Corporation, (1960). See also, Michaels, C. R. and Cane, R. P., "Integral Skin Urethane Foam" S.A.E. Paper 680062, presented at Automotive Engineering Congress -- Detroit, January, 1968, Wirtz, T. "Integral Skin Foam -- A Process in Urethane Molding", paper presented at the Second S.P.I. International Cellular Plastics Conference, November, 1968, Zwolinski, L. M., "Molded Integral Skin Urethane Foam" S.P.E. ANTEC May, 1969, and Grieve, R. L., Bonk, H. W., and Nadeau, H. G., "A Development of Integral Skin Molded Urethane Foams for the Automotive Industry", S.P.E. ANTEC, May, 1969, all of which are incorporated herein by reference.
The difference between integral-skin polyurethane foam and other polyurethane foams is that, during development of the foam in the mold, a skin or noncellular layer is formed, in-situ, adjacent the mold surface that is joined to the interior of the foam through cell walls and ribs. This skin, being integral with the foam, in many cases eliminates the requirement of a separate wear-resistant skin over the finished foam such as is done with other polyurethane foams that do not contain integral-skin.
Difficulty has been encountered in molding integral-skin foam. The integral skin formed in-situ during development of the foam is susceptible to damage from many mold release agents; many of the conventional mold releases inhibit skin development or produce wrinkles and other defects. In addition, the integral skin is quite tender when newly formed, and since many molded foam products are only partially cured in the mold, removal of the foam may cause the skin to tear if less than perfect release is obtained. Further still, many common mold release agents are not very durable and not only must the mold be cleaned and recoated after each molding cycle but the foam product must be cleaned to remove traces of release agent that has plated onto it and that will impair subsequent painting, decorating, or finishing.
The use of polytetrafluoroethylene polymer coatings on mold surfaces is well-known in the art. Furthermore, these polymeric coatings have been used in conjunction with silicone oil mold releases, see U.S. Pat. No. 3,341,646. Still further, waxes have been known to be used in molding, especially polyurethane foam molding, see U.S. Pat. No. 3,101,244. Waxes, however, are softened or melted to effect release and thus require replacement on the mold surface after each molding cycle and also require the molded articles to be solvent washed to remove traces of the wax that has plated thereon. In addition, many mold release agents, while inert to most molding compositions, are acutely attacked by the polyurethane foam reactants, especially the polyisocyanate moiety. Mold release agents that contain (Zerewitinoff) active hydrogens such as hydroxyl groups and amine groups will be affected to varying degrees by these reactive elements in the foam formulation and may well damage the proper development of the foam or cause sticking in the mold. By and large then, the art recognizes not only the difficulties involved in preparing molds for molding polyurethane polymers but recognizes the singularity of mold release problems vis-a-vis specific types of polyurethanes such as elastomers, foams, etc., see U.S. Pat. No. 3,413,390.
In addition to the problem of providing the right kind of mold release agent, other factors influence the situation. Integral skin polyurethane foam uses little, if any, water in the formulations, the main blowing agent being a low boiling hydrocarbon compound such as Freon. Also, integral-skin foam is formulated to a higher density than non-integral skin foam, specifically to provide sufficient polymer for the formation of the skin. These formulation factors make integral-skin foam very costly with respect to materials. To maintain the overall cost competitive with non-skin foam, molding costs must be minimized. This means that mold preparation, molding cycle time, and foam clean-up be reduced. Thus, a mold release agent that must be replaced after each molding cycle cannot be tolerated, nor can a mold release agent that must be additionally heated to be softened or melted to effect release. Mold release agents that plate out heavily on the molded product and require solvent washing also cannot be tolerated. The art heretofore has not found a mold release that satisfies all these aforementioned problems and thus integral-skin polyurethane foam has not found wide commercial acceptance.
This invention is a method of preparing a mold to release flexible polyurethane integral-skin foam in a near perfect surface condition wherein the mold release agent is sufficiently durable to permit up to ten mold cycles prior to renewing. Furthermore, the mold release agent of this invention does not require softening or melting to effect release and does not break away from the mold surface and stick to the integral skin of the foam. Moreover, the minuscule amount of release agent that does adhere to the skin is conveniently washed away from the product by a mere warm water flushing.
Therefore, the main object of this invention is a method of preparing a mold surface that will release flexible polyurethane integral-skin foam therefrom without pinholes or other damage to the skin. Other objects of the invention include a mold release composition that is sufficiently durable to permit a plurality of molding cycles to be accomplished before it needs replaced; a method that permits molding of integral-skin polyurethane foam such that the minuscule traces of mold release that adhere to the molded article may be easily washed therefrom with water; and a method that does not inhibit or interfere with the proper development of the integral-skin foam on the molded polyurethane article.